Besides weighing the powder charge, see if you can identify it by measuring the length and diameter of the kernels and comparing with known types. Could be that it was loaded with a safe load of one type but the wrong type was loaded in the hopper.

Rob, This issue has nothing to do with the propellant. It is simply a case of improperly resized cases which effectively yielded a headspace that was too tight. H.P. White Labs performed a study some years ago that documented that a case that was force fit into a chamber results in excessively high chamber pressures. Basically, the case is deformed by chambering to the point where the case mouth exerts excessive force on the projectile. This results in very high pressure before the projectile is released from the case mouth.